Optical amplifier and optical transmission path

ABSTRACT

The regulated optical amplifiers (V) for wavelength-division multiplex signal transmission respectively comprise a first control device (OE 1 , OE 2 , R 1 ) for regulating the gain and a second, dominating control device (OE 2 , R 2 , R 1 ) having a significantly slower control behavior for regulating the output level (P OUT ) in conformity with a supplied rated value (P OUT ). Both rapid level changes as well as slow changes in attenuation of the transmission path can be leveled given a transmission path equipped with these amplifiers.

[0001] The invention is directed to regulated optical amplifiers and optical transmission paths wherein these amplifiers are utilized.

[0002] Optical amplifiers are utilized in optical transmission networks as compensation for the fibre attenuation. A stable operation over further transmission paths, however, is only possible when modifications of the system parameters are compensated with the assistance of controls. The aggregate output powers of the amplifiers are regulated in previously employed wavelength-division multiplex transmission systems. Given single-channel systems or paths having a constant number of channels, slow changes of the system parameters—for example, due to temperature fluctuation or aging—are compensated rather well with this control concept.

[0003] When, however, the number of channels changes during operation, then such an aggregate output power regulation changes the levels of the individual WDM transmission channels. Such a level change can be fundamentally avoided in that the plurality of active channels is identified and the control devices of the individual optical amplifiers are informed thereof. These correspondingly adapt the rated value for the output level of the amplifier. Due to the different time constants, however, this level matching does not usually succeed without brief-duration fluctuations—considerable losses of transmission quality are connected therewith.

[0004] Another possible solution is comprised in regulating the individual amplifiers to constant gain (amplification). Such amplifiers are described in “Electronic Letters”, Mar. 26, 1991, Vol. 27, No. 7, pages 560-561 and in “Electronic Letters” Jun. 9, 1994, Vol. 30, No. 12, pages 962-964. In these circuits, the level fluctuations of the remaining channels given a change in the number of active channels are suppressed in that the gain is kept constant. However, this control principle is also not suitable as a control concept for a transmission path having a plurality of amplifiers because slow changes in the path parameter sum up and the transmission quality is deteriorated as a result thereof.

[0005] It is therefore an object of the invention to specify suitable amplifiers for optical transmission networks. Moreover, the transmission paths are to be fashioned such that the reception levels of the individual WDM channels remain constant even given a change in the number of channels.

[0006] This object is achieved by a regulated optical amplifier according to claim 1. A version of the amplifier is recited in independent claim 3.

[0007] The transmission paths equipped with these amplifiers are described in claims 6 and 7.

[0008] Advantageous developments are respectively described in dependent claims.

[0009] The advantage of the inventive optical amplifier is comprised therein that the gain regulation in the first control circuit works with a short time constant. Changes in the number of active WDM channels therefore have only a minimum effect on the output level. The second control circuit sees to it that slow changes are leveled out. Given a change in the plurality of active channels, a corresponding change of the rated value of the second control circuit ensues on the basis of a local terminal inserted into the transmission path at the transmitter side or on the basis of a terminal (network node) of the reception side, so that the output level control is perceptible only brief-duration and only minimally, if at all.

[0010] When a storage element is provided in the second control circuit, then it is possible to have this control circuit take effect only at specific times in order to modify the reception level or to deactivate it during a change in the number of channels.

[0011] In an alternative solution, the regulation of the output power either ensues in common for all amplifiers of a transmission path proceeding from the reception terminal or, given a corresponding, individual monitoring of the output levels, also ensues separately via a correspondingly fashioned monitoring channel.

[0012] Due to the employment of an output level regulation, the amplifiers need only receive information about the plurality of WDM channels or a corresponding rated value.

[0013] The transmission paths equipped with these amplifiers can also level out slow changes in amplification caused by aging processes.

[0014] The invention is described in greater detail on the basis of two exemplary embodiments.

[0015] Shown are:

[0016]FIG. 1 a schematic circuit diagram of the inventive amplifier with regulation of the output power;

[0017]FIG. 2 a schematic circuit diagram having a fiber amplifier;

[0018]FIG. 3 a transmission path having a plurality of amplifiers;

[0019]FIG. 4 a version of the inventive amplifier; and

[0020]FIG. 5 the use of this amplifier on a transmission path.

[0021] An exemplary embodiment of the invention is shown as a schematic circuit diagram in FIG. 1. An optical amplifier V serves for the amplification of a wavelength-division multiplex signal MS transmitted via a light waveguide LWL. A first measuring coupler K1 that branches a part of the signal off is provided at the input side. This part of the signal is converted by a first optoelectronic transducer OE1 into an electrical measured signal p_(IN) corresponding to the input level (input aggregate power) P_(IN), this measured signal p_(IN) being supplied to a first regulator R1. A measured signal p_(OUT) is likewise acquired via a second measuring coupler K2 and a second optoelectronic transducer OE2, this second measured signal p_(OUT) corresponding to the output power P_(OUT) and being likewise supplied to the first regulator. Dependent on the (adjustable relationship) P_(OUT) to P_(IN), for example, the pump current I_(PUMP) is regulated given a fiber amplifier or, respectively, the control current is regulated given a semiconductor amplifier. Other principles of gain control can likewise be employed, these, for example, being described in the cited references.

[0022] In addition to a first control means (control circuit) (K1, OE1, K2, OE2, R1, V) serving the purpose of a rapid gain control and shown in simplified fashion, a second, dominating control means (control circuit K2, OE2, R2, R1, V) is provided, this regulating the output power (output level) P_(OUT) by comparing the corresponding measured value p_(OUT) to a reference input, the rated value p_(RATED). Slow changes of the transmission attenuation caused, for example, by temperature modification or aging are leveled by this second control circuit. The manipulated variable G_(RATED) output by the second regulator R2 defines the pump current by intervening in the first control circuit and, thus, defines the gain of the optical amplifier. Given changes in the number of transmission channels, the gain should not change. The level regulation therefore dare not take effect immediately, this being capable of being achieved by a far, far higher time constant of the second control circuit, as a rule, compared to the time constant of the first control circuit.

[0023]FIG. 2 shows details of the amplifier circuit with a fiber amplifier VFA whose gain is defined by the pump current I_(PUMP) generated by a controlled pump laser PL, this being fed in via a pump coupler PK. The first regulator R1 can contain an attenuation element DG that is connected to the second opto electrical transducer OE2 and can contain a first comparator COM1. When the second control circuit is left out of consideration, then the gain can be set with the attenuation element. One possibility for “output level regulation by modification of the gain” would be the direct change of the attenuation element DG by the reference input p_(RATED).

[0024] As already fundamentally described, the comparison between the output power and the reference input P_(RATED) in the exemplary embodiment ensues in a second comparator COM2 in the second control means (in the second control circuit) K2, OE2, COM2, MU, IN, COM1, PL, PK, VFA). Via the multiplier MU, the result of this comparison changes the input signal of the first comparator COM1 and thus controls the pump current and, thus, the gain of the fiber amplifier VFA. The attenuation element can be foregone since the second control circuit determines the gain via the multiplier.

[0025] As already mentioned, the time constant in the second control circuit should be adequately great in order, when the number of channels is changed, to be able to neutralize the influence thereof on the basis of a corresponding, externally implemented change of the reference input. A storage element SH can likewise contribute thereto. This can also be inserted between the integrator and the multiplier. A range from approximately 1 microsecond up to 1 millisecond is adequate for the first control circuit as time constant given high data rates in the megabits per second range, and a range from approximately 0.1 seconds over several seconds and minutes up to hours is expedient for the second control means. The time constant can also be switched for different operating conditions. For the initialization, thus, a low time constant of, for example, 100 microseconds can be selected; a time constant of 1 second can be selected given a change in the number of pluralities; and a time constant of several minutes can be expedient given a level change ensuing as needed.

[0026] An integral behavior or at least an integral part that can also be supplemented by a dead time lies at hand for the second regulator. The second comparator and the integrator can be combined in one circuit embodiment.

[0027] The amplifier circuits with the appertaining control circuits can, of course, be constructed in any desired way.

[0028]FIG. 3 shows a transmission path with a plurality of optical amplifiers VT, V1 through Vn. A wavelength-division multiplex signal MS is generated in a transmission terminal Ti in a transmission means TR having a following wavelength-division multiplex WDM, is amplified in an optical amplifier VT and is supplied into the path. The amplifiers are set such that they respectively supply output levels corresponding to the conditions of the respective path section, these likewise being maintained by the second control circuit even given slowly changing transmission properties.

[0029] When the plurality of WDM channels is varied, then the output level in each channel continues to be held constant at first by the first control circuit. Due to the slow time constant/dead time, the output level regulation does not initially intervene in the regulation process. Since the amplifiers are simultaneously informed of the change of the reference input by the terminal via a monitoring channel OCH, this change of the reference input serving for setting the new output level, practically no influencing due to the second control circuit ensues. A separate monitoring of the number of channels allocated to each amplifier is still too complex compared thereto.

[0030] It should also be added that the output powers can also be individually adjustable via the monitoring channel.

[0031]FIG. 4 shows a version VV of the inventive amplifier. The second regulating means for direct regulation of the output level is lacking. The gain can only be set via the monitoring channel OCH in order to then be kept constant by the first control circuit. An external modification of the output level is also in turn possible. In this example, the setting ensues via a digital-to-analog converter DAW whose output signal determines the gain as control signal G_(RATED).

[0032]FIG. 5 shows a further transmission path having optical amplifiers VV1 through VVn wherein this type of amplifier can be advantageously utilized. A terminal T2 of the reception side, in addition to containing an amplifier VVn and a wavelength-division demultiplexer WDD, contains a reception means RE that determines the aggregate level and the plurality of active WDM channels. Proceeding from the second terminal, the numbers of channels or, respectively, corresponding rated values are communicated to the amplifiers via the transmission channel OCH, as is the extent to which the gain of the individual amplifiers is varied as well given system-induced, slow changes of the reception level. The second “control circuit” is thus always formed via the reception terminal. Given simple embodiments of the setting device and of the amplifiers, identical variations in gain can be undertaken for all amplifiers; given more complicated embodiments, individual variations at individual amplifiers can be undertaken according to the path parameters or can be accordingly undertaken by monitoring devices. The output levels can also be directly identified or be modified in relationship to preset levels on the basis of correspondingly implemented regulators. 

1. Regulated optical amplifier (V, V1, . . . ), particularly for wavelength-division multiplex signal transmission, comprising a first control device (V, OE1, OE2, R1) for regulating the gain, characterized in that a second, dominating control device (V, OE, R2, R1) having significantly slower control behavior is provided for regulating the output level (P_(OUT)) according to a supplied reference input (P_(RATED)).
 2. Regulated optical amplifier (V, V1, . . . ) according to claim 1, characterized in that the first control device is formed of an optical amplifier (PL, PK, VFA), a second opto electrical transducer (OE2) connected to the output thereof and a first comparator (COM1) to which a measured signal (p_(IN)) corresponding to the input level is supplied via a first opto electrical transducer (OE1); and in that the second control device is essentially formed of the optical amplifier (PL, PK, VFA), the opto electrical transducer (OE2) connected to the output thereof, a second comparator (COM2) to whose second input a rated value (P_(SOLL)) is supplied, a multiplier (MU) inserted between the first opto electrical transducer (OE1) and the first comparator (COM1), the output signal of the second comparator being supplied to said multiplier, and of the first comparator (COM1).
 3. Regulated optical amplifier (VV, VV1, . . . ), particularly for wavelength-division multiplex signal transmission, comprising a first control device (OE1, OE2, R1) for regulating the gain, characterized in that a control device (DAW, IN, MU) is provided with which the output power (P_(OUT)) can be set by setting the gain according to a rated value (P_(RATED)/G_(RATED)) supplied as reference input.
 4. Regulated optical amplifier (VV, VV1, . . ) according to claim 3, characterized in that the first control device is essentially composed of an optical amplifier (PL, PK, VFA), a second opto electrical transducer (OE2) connected to the output thereof and of a first comparator (COM1) to which a measured signal (p_(IN)) corresponding to the input level is supplied via a first opto electrical transducer (OE1); and in that the control device (DAW, IN, MU) contains a multiplier (MU) to which a regulating signal (G_(RATED)) corresponding to the externally supplied rated value (p_(RATED)) is supplied.
 5. Regulated optical amplifier (V, VV) according to one of the preceding claims, characterized in that said amplifier is implemented as a fiber amplifier (PL, PK, VFA); and in that the pump current (I_(PUMP)) is controlled by the control devices.
 6. Transmission path comprising a plurality of amplifiers (V, VV1, . . . ) connected in cascade, characterized in that a rated value (P_(RATED)) determining the respectively desired output level (P_(OUT)) is supplied to the amplifiers (V, VV1, . . . ) as reference input.
 7. Transmission path comprising a plurality of optical amplifiers (VV) according to claim 3, 4 or 4 and 5 connected in cascade, characterized in that a reception terminal (T2) is provided that monitors both the aggregate level at the reception side as well as the plurality of active WDM channels and, given variation of the aggregate level and a constant plurality of WDM channels, readjusts the gain of the optical amplifiers (VV1, . . . , VVn) as part of the second control device.
 8. Transmission path according to claim 6 or 7, characterized in that a digital rated value (P_(RATED)) that contains the plurality of active WDM signals is transmitted as reference input.
 9. Regulated optical amplifier (V, VV) according to claim 1, 2, 3 or 4, characterized in that the second control device )OE, R2, R1; RE, DAW, MU, COM1, PL, VFA) comprises an integral part or is fashioned as integral regulator.
 10. Regulated optical amplifier (V, V1, . . . ) according to claim 1, 2, 3 or 4, characterized in that the second control devices comprises a dead time part.
 11. Regulated optical amplifier (V, V1, . . . ) according to claim 1, 2, 3 or, [sic] characterized in that the time constant of the second control device (OE, R2, R1; RE, DAW, IN, MU, COM1, PL, VFA) can be switched. 